13" F/3 telescope build

[kb58]

The secondary is attached to the secondary frame by means of the 1/4-20 threaded rod in the center, there are 3 points from the frame that hit the center drilled holes in the secondary holder. These 3 (4-40 screws with pointed ends) are used to point the secondary when collimating the scope.

Thanks for the reply! I wasn't clear in my first question: what adhesive did you use to attach the glass secondary to its holder?

 

[Mitch Alsup]

Thanks for the reply! I wasn't clear in my first question: what adhesive did you use to attach the glass secondary to its holder?

post #56

 

[kb58]

I used silicone on a 10" mirror and it was a real ***** to remove. That's why I was asking, and surprised that some people said their mirrors were dropping off.

 

[Mitch Alsup]

Removal requires a razor blade (and it better be single edged!) and some acetone.

I have used the 3.8" on my 20" F/4 DOB for 19 years without any hint of failure. I'm just glad the coating is holding up as well as it is.

The mirror is currently sitting on 3 sheets of tissue paper, a wooden log held in the vicce on my mill with the aforeseen part held square at 0.038" above the secondary with silicone between the two parts curing.

 

[Mitch Alsup]

I got out and had first <day>light with the scope this afternoon.
I can reach focus on objects that are under 200-odd feet away but not farther.
This tells me the truss poles are too long. So I bobbed them by 1/2".

But what I thought you guys might like to see is:: "How does one get 6 poles to the same length, within a couple of thou, when one does not have a measuring device that long?

I came up with this::



Edge to edge of my tables aw is within spitting distance of 27", and the newly bobbed poles are trying to be 26.5" (first try).

So I got out my mill setup stuff and a couple of 1-2-3 blocks and clamped them to the table saw bed.
Then I took a random piece of 1/2"×1" mild steel tubing and clamped it to a 1-2-3 block.
Now I can set the pole lengths the same way one feels a telescoping gauge in the micrometer.
The poles are in a circular chain, so one simply takes the one on top as the next one, does the feel test,
and when all 6 touch both the 1-2-3 block and the steel tube, with that micrometer feel, then they are all the same length.

 

[Mitch Alsup]

Here is the packaged scope fitting the the back of my Merc S-600. And if anyone knows about the size of the trunk in this car, you will be amazed at how SMALL it is, especially with the refrigerator in the back.

 

[Mitch Alsup]

Someone ask about the scale of this "thing", so here is a picture with a 24" ruler/level::



It is actually 21" tall in storage, 16.5" if you don't count the altitude bearings. And, yes, that is the whole scope (i.e., all the necessary pieces.)

 

[Mitch Alsup]

Anyway, how are you planning to attach the secondary to its support? I real of all kinds of methods, and most work, and a few don't! I haven't decided on the spider design. At first I was heading down the wire rabbit hole but may end up doing very much the same as you for rigidity and simplicity.

See #37 in this thread for the vanes and secondary frame.

In my 20" F/4 I spent 2 years fighting 3 different wire spider designs never getting rid of vibrations or collimation issues. Then I gave up and built a 4-vane spider using 0.007,5 galvanized steel (roof flashing; $10 at Home Despot for a lifetime supply). This thing worked so well I never needed anything better. The new 13" F/3 has a similar 4-vane spider (the design shown in the link below (which is just page 1 of this thread). It is strong enough you can grad the whole upper assembly by the secondary frame and pick up the upper assembly forcefully without harming collimation.

Here is a picture of the secondary attachment double sticky taped to the secondary mirror:



This bracket has:
a) slots cut into it to relieve some of the thermal differences in expansion between AL and Glass
b) kinematic mounting points on the secondary frame to preserve most of the collimation from mounting and dismounting
c) articulates all forces through the CoG of the secondary (see:: https://www.hobby-machinist.com/threads/13-f-3-telescope-build.65805)
d) the secondary lives in a box (like an EP box) while the scope is not being used (for protection of both the secondary and primary)

I used a similar model for my 3.8" secondary of my 20" F/4 that I have used for 20-odd years.

Here is a picture of the secondary sitting on 5 sheets of tissue paper, sitting on a board in the milling vise as the silicone cures. I am using the quill and spindle to hold the secondary bracket square to the secondary as the silicone cures, and using the quill height to set the proper thickness of the silicone blob. Whilecuring I wrapped this whole thing with 5 layers of more tissue paper to prevent crap.....

Regarding the ball ends on your trusses, did you fabricate them or buy knobs from McMaster? Of the latter, are they aluminum?

If I recall correctly, I got them from "cheap balls" 1" 6061 balls with 1/4-20 threaded holes.
All I did was spin them up on the lathe with 1500 grit paper to polish them.

When complete, mine will be a 16" f/4.3 6-truss design. The details of its aesthetics is still up in the air, but will probably end up more functional than artistic. I don't plan on transporting it much and want it stout and not losing collimation too easily.

I am hoping the mechanism I built will hold collimation through disassembly and reassembly!

I collimate the scope by changing the length of the poles. The balls were drilled and tapped for 4-40 set screw that holds the 1/4-20 threaded rod (4140) in the ball. The pole end is threaded 1/4-20, and the tube has a 1/4-20 star nut inside.

A 6-pole truss with adjustable pole lengths is known as a "Stewart Platform" and has several important properties:
a) 6 degrees of freedom supported by exactly 6 points (the balls in the ball races.)
b) adjusting pole lengths allows the upper assembly to translate and tip/tilt/droop in 3 dimensions.
b.1) I collimate with a laser by translating the upper assembly so the laser comes back to the secondary where it originally left the secondary.
b.2) this is why the poles all start off the same length (no tip/tilt/droop)
c) once collimated, the pole end is tightened and the ball race clamp is tightened and the scope remains collimated
c.1) while at the same time the truss is converted from pivoting ends to fixed ends (for evenmore stiffness.)
c.2) once tightened all one has to do is put the poles back where they were and it will be "close" probably very close (we shall see.)

 

[Mitch Alsup]

A dupe of the previous.

 

[Mitch Alsup]

I thought some might want to see the arrangement I used to square the upper assembly to the lower assembly::

The first thing to do is to square the lower assembly to gravity. So I took the scope off its base and place it on plywood on top of the table saw. Then I shimmed this until it was square to gravity in 2 orthogonal directions.

Next the poles and the upper assembly are assembled. On the 4 spider vane attachments, 4-plumb bobs (darts) are positioned to almost touch the lower assembly.

Then the upper assembly is translated using pairs of poles until the plumb bobs indicate the upper assembly is square with the lower assembly. At which point I remembered to take a picture::



Then individual poles were lengthened until the upper assembly was square to gravity using a level in a 24" rule.



At this point the upper assembly is square to the base. I expect to have to lengthen the poles (end to end) but I am rebuilding the "ground board" for more comfortable use of the scope (the poles need to be longer--which I did today.)